When an airline denies you a seat you've reserved because it has oversold the flight, it typically offers you a voucher for the inconvenience. But there are two things you need to know: Under U.S. Department of Transportation rules, you are entitled to insist on money instead, and as of May 19, airlines must pay you up to twice as much as previously required.

More specifically, if you are involuntarily bumped from a domestic flight and the substitute flight offered is scheduled to arrive between one and two hours later than your original flight, you soon will be entitled to a refund of your one-way fare, up to $400. If the new flight is scheduled to make you more than two hours late, payment jumps to as much as $800.

For international flights, the airlines are given more time: You can claim up to $400 if the substitute flight is scheduled to arrive within one to four hours of the original. If it's more than four hours, you get $800.

If you choose not to take an alternative flight, you are entitled to a full refund plus the compensation.

Before bumping a passenger involuntarily, airlines must ask for volunteers, and they typically offer vouchers for a free ticket or a given amount off your next flight. This column is guessing that the new rules about involuntary bumping will encourage airlines to be more generous with offers to volunteers. If you do volunteer, check restrictions on vouchers, know when you'll get an alternative flight and, if the delay is long, find out whether food and shelter will be provided.

And remember: This compensation applies only if you're bumped, and certain very small planes are exempt. You may be equally inconvenienced by weather, in which case the airline owes you nothing. If you're inconvenienced by mechanical problems or other factors within an airline's control, you are owed whatever the airline promises in its contract of carriage, which usually is not much.

Book passage now to save

Pondering a Disney Cruise? You can avoid a hefty new fuel surcharge if you book by May 27. The fee for cruises booked after that date is $8 per passenger per day for the first two passengers in a cabin and $3 per day for additional passengers in the same cabin.

Disney was the last of the major cruise lines to respond to higher fuel prices with surcharges. Most imposed them late last year and upped the amount recently. First time around, some lines ran afoul of the Florida attorney general by imposing the fee on passengers who had booked before the surcharge was announced. Those passengers are owed refunds of the surcharge amount.

All of the lines set a surcharge cap for long cruises: Disney's surcharge, for example, is capped at $112 for each of the first two passengers in a cabin ($224 per couple) and $42 for each additional roommate. In all cases, the surcharge is less for the third person in a cabin.

A quick rundown:

• Celebrity and Royal Caribbean have increased their fuel surcharges from $5 to $8 per person per day. If you booked a 2008 cruise before the first announcement of a surcharge on Nov. 16 and paid the extra fee, ask for a surcharge refund.
• Norwegian has increased its daily fuel surcharge from $7 to $9 per person.
• Carnival, Costa, Cunard, Holland America, Princess and Seabourn have raised their fuel surcharges from $5 to $7 per person per day. If you booked a 2008 cruise before the announcement of the surcharges on Nov. 7 and paid the fee, ask for a refund.

Second bag charges

No use getting all huffy if you find yourself being charged for checking a second bag. The charge is now standard among major U.S. carriers, except for Southwest.

American becomes the last of the U.S. legacy carriers to announce a $25 charge for a second bag for domestic passengers with economy tickets. United, the first to announce the charge, got a lot of flak but was followed by Continental, Northwest, Delta and US Airways (whose fee also applies to international flights).

All six exempt some travelers -- including active-duty military and frequent fliers who have flown enough to earn special status -- and different start dates apply. Generally, though, you can still check up to 50 pounds free, but everything must fit in one bag. The fee for "domestic" flights also may apply to flights to Canada and U.S. territories. Check the airlines' Web sites for details.

Meanwhile, JetBlue will charge all passengers $20 to check a second bag as of June 1.

Jet fuel prices have skyrocketed. If airlines must charge more for something, optional charges are better than general price increases that passengers can't choose to avoid.